Asymmetric [4 + 2] cycloaddition synthesis of 4 H -chromene derivatives facilitated by group-assisted-purification (GAP) chemistry.

In this work, we present a strategy for the preparation of functionalized 4 H -chromene derivatives via a Cs 2 CO 3 -catalyzed [4 + 2] cycloaddition of enantiopure chiral salicyl N -phosphonyl imines with allenoates. Fifteen examples were achieved in excellent yields and diastereoselectivity. The products were purified simply by washing the crude mixture with hexanes following the Group-Assisted Purification (GAP) chemistry/technology to bypass traditional separation methods. The absolute configuration was unambiguously determined by X-ray structure analysis.